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of consciousness would be associated with a unique combination of these four attributes. For example, dreaming sleep is induced by going to bed, closing one's eyes, and counting sheep; by subjective reports of a lapse in consciousness or dreaming; by observable behaviors such as closed eyes, prone position, and slow breathing; and by high-frequency, low-amplitude, desynchronized brain waves accompanied by rapid, synchronous eye movements. However, such clear specification of the four features does not characterize most altered states of consciousness.

In some instances, this lack of clear specification reflects the state of current technology and incomplete knowledge from available experimental work. But it is also not clear that the relationship between mind and brain is such that it will ever be possible to specify unique psychophysiological correlates of different states of consciousness. Accordingly, in this chapter, we consider the effects on human performance of a number of conditions that are conventionally defined as altered states of consciousness. Chief among these is hypnosis, a technique that has been widely used in attempts to enhance human performance. We also discuss restricted environmental stimulation and update the committee's previous reviews of sleep learning and meditation (see Druckman and Swets, 1988:Ch.4; Druckman and Bjork, 1991 :Ch.7).

HYPNOSIS

Hypnosis is a social interaction in which one person, a hypnotist, offers suggestions to another person, a subject, for experiences involving alterations in perception, memory, and the voluntary control of action. Hypnosis is typically induced by suggestions for relaxation, focused attention, and closing one's eyes. After a subject's eyes have closed, the procedure continues with suggestions for various sorts of imaginative experiences. The range of such experiences is very broad: a hypnotist may ask a subject to extend his arm and suggest that he is holding a very heavy object, whose weight is pressing the hand and arm down; a hypnotist may ask a subject to interlock her fingers and suggest that her hands are glued together so that they cannot be pulled apart; a hypnotist may suggest that there is a voice asking questions over a loudspeaker, to which the subject should reply; a hypnotist may suggest that a subject cannot smell an odorous substance held near his nose; or a hypnotist may suggest that a subject is growing younger and reliving an experience from early childhood. A subject may also be given a posthypnotic suggestion: for example, that after hypnosis, when the hypnotist picks up a pencil, the subject will stand up, stretch, and change chairs—but forget that he had been told to do so. A subject may also receive a suggestion that upon awakening she will be unable to remember the events and experiences that transpired during the hypnotic session;